Passive Transport

Passive transport is different than active transport because passive transport allows molecules to move across membranes without the need for energy.

Active transport requires some form of energy to move across membranes.

All types of cells have some form of transport so that they can move molecules and substances from one area of the cell to another to help to keep a balance.

Passive transport rate depends upon how permeable the cell membrane is and that depends on the characteristics and organization of the proteins and lipids.

There are four different types of passive transport: diffusion, facilitated diffusion, filtration, and osmosis.


This movement relocates ions, molecules, and substances from an area of high concentration to a low concentration area that needs them.

The movement continues along a concentration gradient, back and forth across the membrane, until each side has the same amounts.

This type of movement doesn’t require any energy to accomplish the final goal. Once the “concentration gradient” is eliminated, there won’t be any further transport.

The importance of diffusion is that it gets rid of concentration gradients that are in the body.

An example of this might be when we breathe in air that allows oxygen into the blood stream, the metabolic activity then uses the oxygen when the alveoli in our lungs diffuses it.

Once diffused, the lungs can then get more oxygen with the next breathe.

Facilitated Diffusion

This type of passive transport is also called “carrier-mediated osmosis.”

It involves the use of a special protein that is part of the membrane that allows molecules to move across the membrane.

Because the proteins are already there and have a specific purpose for the movement, no energy is required for this transport.

Facilitated diffusion is the perfect answer when there are larger molecules such as lipids that are not soluble or even glucose, that are too big to fit through the standard pores of the membrane.

This is when the special carrier proteins step in and they bond to a receptor site and move the molecule through the membrane.

The process makes use of movement down the concentration gradient, moving from high concentration areas to low concentration areas.


This process uses water movement and solute molecules to move across the membrane of the cell with hydrostatic pressure.

There are limits in this transport in the size of the pores of the membranes. Only certain sizes can pass through.

Examples of this are some of the pores in the kidneys that are so tiny that only the littlest of proteins can be moved through.

However, liver cell pores are very large and they allow a number of the solutes to move through so that they can be metabolized. None of these actions require any energy to complete.


This is the diffusion of water molecules across a membrane. Almost all cell membranes are water permeable.

The molecules of water stick together with weak hydrogen bonds this makes water molecules have the ability to move in clumps and this is called “bulk flow.”

The movement itself balances each area’s quantity of water. Once it achieves a balance in water concentration in both areas, it achieves a condition called “isotonic” and the water stops moving.

The opposite of isotonic is “hypotonic,” and this is where there are unequal concentrations. Following the concentration gradient, the water will move through the membrane from high to low concentration areas.

Facts about Passive Transport

  • Some cells function better at different pH values ranging from 5-7.
  • In facilitated diffusion the transporter channel GLUT4 is important in controlling diabetes. It helps to insert just the right amount of glucose so that the sugar that enters our cells is lower than the actual amount of sugar that we consume.
  • When gas is exchanged in photosynthesis it is an act of diffusion in passive transport. The carbon dioxide flows from leaf to air/air to leaf.
  • Passive transport using filtration can be seen in both the kidney and renal filtration when the body’s blood is filtered in the kidney area called the “glomerulus,” and retaining some to be reabsorbed.
  • Osmosis allows liquids to enter the cells so that they can be used by the body and they pass through semipermeable membranes that only allow the fluids and not the solutes.
  • An example of passive transport diffusion that we use every day include making medicines easier to take because the drugs inside the capsule can be moved into the digestive area where diffusion moves them into the bloodstream.

Interesting Facts about Biology

  • Malaria is a disease that is caused by a protest.
  • Scientific study shows that the liver accomplishes as many as 500 functions.
  • Empty calories are the foods that we eat that have little or no nutritional value but do have calories. The most common of these are added sugars and solid fats.
  • The second time your body is exposed to an antigen it responds more quickly.
  • In 1869, Freidrich Meischer, a Swiss biologist, isolated and identified DNA.
  • Your lymph nodes are home to both T and B cells and these are throughout your body. Their job is to enter the blood stream to help fight off diseases.

Passive Transport Quiz

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